Ablative and insulative structures

ABSTRACT

An ablative structure in the form of a relatively thick-walled body of revolution is prepared by laying up, substantially sideby-side successive panels of high temperature resistant materials impregnated with a thermally curable resin within a female mold, disposing between certain panels removable aperture-defining plugs out of communication with the interior and exterior surface of the body, curing the resin and densifying the structure under radially outwardly exerted pressure and removing the plugs from the structure.

United States Patent Donald M. Hatch Harbor City, Calif. 679,281

[72] Inventor [21] Appl. No. [22] Filed Sept. 27, 1967 [45] Patented May 11, 1971 [73] Assignee Hitco Division 01 Ser. No. 450,076, Apr. 27, 1965, Pat. N0. 3,449,189.

[54] ABLATIVE AND INSULATIVE STRUCTURES 11 Claims, 9 Drawing Figs.

52 u.s.c1 138/141, 60/356, 220/83, 102/105, 138/149, 60/208, 29/157 51 lm.Cl F16l9/l4 so FieldofSearch 138/141,

[56] References Cited UNITED STATES PATENTS 4/ 1952 Smith 102/ 105 Primary Examinerl .aveme Dv Geiger Assistant Examiner-Henry K. Artis Attorney-Fraser & Bogucki ABSTRACT: An ablative structure in the form of a relatively thick-walled body of revolution is prepared by laying up, substantially side-by-side successive panels of high temperature resistant materials impregnated with a thermally curable resin within a female mold, disposing between certain panels removable aperture-defining plugs out of communication with the interior and exterior surface of the body, curing the resin and densifying the structure under radially outwardly exerted pressure and removing the plugs from the structure.

PATENTEU mm 1 IHYI 3, 578,030

SIIZET 1 OF 3 FIG.4

INVENTOR.

ABLATING GASES F |G 2 ATTORNEYS FMMB KJM- 00mm n. men

PATENTED HAY! 1 IHTI SHEET 2 OF 3 FlG.-6

Wi? Q FlG.-7

ATTORNEYS PATENTED MAY] 1 l97| mvsw'ron. DONALD M. HATCH A TTORNE Y8 ABLATWE AND WSULATIVE STRUCTURES Advancements in high temperature technology have l0 resulted in the development of materials and structures for withstanding severe environments for adequately long time intervals. Such structures are often required to withstand not only high temperatures, but also the eroding effects of gases moving at high velocities. Any useful system operating under these conditions must provide an adequate degree of thermal insulation, as well as protection from erosion. Thus, although certain metals and alloys are known which can withstand both high temperature and gas velocities, the heat conductivities of these materials are such that the cold side temperature very quickly becomes excessive.

Other expedients have therefore been adopted or experimented with, in attempts to overcome the multiple problems presented by such environments. These expedients have included the use of cooling media adjacent to or moving along the exposed surface, and the use of materials or structures which provide protection of the exterior surface. By far the most widely used technique, however, is based upon the principle of ablation. in accordance with the severity of temperature and gas velocity to be encountered, and the length of time for which protection is required, the ablative surface erodes relatively slowly and in a controlled fashion, while still presenting a high degree of resistance to the transfer of heat. Thus, as the ablative surface is progressively eroded, it must remain effective for the dual purposes of structural protection and insulation throughout the entire operating interval of the system with which it is employed. The ablative structure typically is a composite structure formed of a high temperature resin matrix reinforced by selected fiber materials. Resistance to erosion is primarily determined to a considerable extent by the composition and properties of the reinforcing fibers. The fibers often are most conveniently handled in the form of preimpregnated fabrics or tapes, built up into the desired solid shape. The entire structure is usually densified in a hydroclave or autoclave as the resin is cured at elevated temperatures.

As increasingly stringent demands have been made on ablative structures, it has been found that superior ablative and insulative properties are achieved by selective control of the orientation of individual fibers. Longer life and better control are achieved if the individual fibers are disposed such that they erode progressively along their length. For this purpose, the ends of at least most of the fibers are usually oriented at a substantial angle to the exposed surface. At the same time, heat conduction along the fibers is apt to be the largest contributing factor to heat conduction in the insulative structure, so that it is highly desirable that a long path exist between the exposed surface and the cold side along the fibers, even though the structure may have relatively thin walls. Another important consideration is that the orientations of the tapes or fabrics should be such as to avoid delamination as the exposed surface erodes.

It has been found that superior combinations of ablative and physical properties are derived by the formation of multi-ply composite bodies from a plurality of individual panels, each disposed at an oblique angle relative to the exposed surface, and each extending from the exposed surface to the opposite cold side surface along an extended path that forms an are between the exposed and opposite surfaces. In this manner, the fiber orientation may be optimized and the length of the conductive path greatly increased. In addition, with this type of configuration, there are material advantages in uniformity and in versatility, inasmuch as a number of different types of material may be combined into a uniform and unified structure, to optimize the characteristics of the unit.

As more stringent demands are imposed on such structures, it becomes necessary further to improve their insulative and structural properties. An ablative body is typically in the form of a hollow surface of revolution and the exposed surface is usually eroded to a given extent, generally termed the char depth. The material within the structure on the opposite side from the exposed surface forms a heat sink as well as mechanical reinforcement. It is highly desirable to enhance the heat sink characteristic without using a greater amount of material or to retain the same insulative properties while reducing the weight of the structure. It is additionally highly desirable to be able to modify the complex so as to permit particular structural variations, as for example, the handling of a cooling medium.

it is therefore an object of the present invention to provide improved high temperature structures.

A further object of the present invention is to provide improved ablative insulative structures having lower weight for given structural and insulative characteristics than has heretofore been feasible.

Yet another object of the present invention is to provide improved ablative structures which require less material than prior art structures but provide greater insulative properties and are capable of many variations in configuration.

Another object of the present invention is to provide improved composite bodies for ablative and insulative applications and having lighter weight than existing structures.

A further object of the invention is to provide improved methods for fabricating high temperature bodies.

These and other objects are achieved by structures and methods in accordance with the invention which introduce fluted or hollow interior volumes within the structure. The interior flutings are so introduced that they do not materially decrease the structural properties, while enhancing the heat barrier presented by the structure.

In accordance with one aspect of the invention, an internally fluted ablative structure is provided from a multiplicity of individual panels laid up to form a hollow surface of revolution. Each panel lies along an arcuate path between the internal and 'extemal surface of the body, and has a length between these two surfaces which comprises a substantial part of the circumference of the body. The body is solid for a given depth from the exposed surface in accordance with the needed char depths. in the central part of the body wall, however, the panels are curved or spaced by other panels so as to define the interior flutings. The flutings typically do not necessarily extend along the full length of the body. The flutings may be evacuated or filled with low density materials for specific heat transfer and strength characteristics. In any event, the presence of the fiutings effectively lengthens the heat conduction path extending from the exposed surface to the cold side surface along the individual plies. Alternatively, a cooling medium may be passed within the flutings where requirements dictate or system operation is improved.

Methods in accordance with the invention readily incorporate the desired interior volumes into ablative and insulative structures without requiring special equipment or modification of the structures themselves. During layup of a body forming a hollow surface of revolution by the multi-ply technique, for example, elongated strips of nonadhesive or destructive nature may be inserted within the plies at appropriate spacings around the periphery of the body. The characteristic of this type of ablative structure is that during conventional hydroclave or autoclave densification, adjacent panels move freely relative to each other as the .body compresses, until the ultimate form is reached. Thereafter, the inserts may be removed or decomposed, following densification, leaving an ablative structure with interior fluting of the kind desired.

A better understanding of the invention may be had by referring to the following description, taken in conjunction with the accompanying drawing, in which:

FIG. I is a perspective simplified view, partially broken away, of an ablative structure in the general form of a hollow body of revolution in accordance with the invention;

FIG. 2 is a side sectional view of the arrangement of FIG. 1;

FIG. 3 is an enlarged plan view of a fragment of the arrangement of FIG. I, showing the disposition of panels therein;

FIG. 4 is an enlarged plan view of a fragment corresponding to FIG. 3, showing an alternative disposition of panels therein;

FIG. 5 is an enlarged plan view of a fragment corresponding to FIG. 3, showing a second alternative disposition of panels therein;

FIGS. 6, 7 and 8 show successive steps in a method of con structing an ablative structure in accordance with the invention; and

FIG. 9 is a simplified sectional view of an ablative and insulative system using a cooling medium in conjunction with structures in accordance with the invention.

Referring now to FIGS. 1 and 2, ablative and insulative structures in accordance with the invention are typically formed as thick walled bodies defining surfaces of revolution. For a rocket nozzle 10 as shown, for example, the exposed surface is typically the interior of a truncated cone or other body. The depth of material to be ablated or substantially modified in character constitutes the char depth, and in the rocket nozzle shown is assumed to require (together with the immediately adjacent solid pan needed for backup) approximately one fifth or one-fourth the radial thickness of the nonle 10.

In the multi-ply construction shown, the ablative body is formed of a plurality of individual panels, each having a longitudinal edge contiguous with the external surface, as well as a longitudinal edge contiguous with the internal surface, with the panels being successively overlaid in the circumferential direction. Thus each panel follows a relatively long arcuate path between the inner and outer surfaces and presents only the edge of the panel to the eroding gases.

Here, the versatility of this type of structure is used to advantage, by employing one type of panel at the larger end, and a different type of panel 14 at the smaller end. The two types of panels l2, 14 are interleaved in the intermediate region into a unitary structure. Other variations of this system are also feasible, because different panels may be interleaved through a given thickness, or through separate adjacent thicknesses.

If the panels are composed of fabric materials, the fabric weave and cut may be controlled so that the individual fibers have optimum achievable orientations for desired properties. In practice, this type of structure offers many advantages over tape wound and other ablative structures because it is readily fabricated, has a high degree of uniformity, and is capable of many modifications. The panels may be laid up rapidly within a female mold, for example, and various materials may be interleaved in regular patterns. The panels may be varied either or both longitudinally and circumferentially, to tailor strength and temperature characteristics to desired properties along the length and throughout the thickness of the nozzle 10. Because the panels are free to slide relative to each other during densification and curing, the resultant structure is free of wrinkles and undesired voids. All such advantages are retained in configurations in accordance with the invention, although it is not necessary to show the various modifications of fonn or material which are feasible.

According to the form of the invention shown in FIGS. 1 and 2, the nozzle 10 is completely solid to a given depth from the exposed surface, but the panels are slightly distorted through curvature to define interior flutings 16 or apertures at successive areas about the periphery of the nozzle 10. As best seen in FIG. 3, the panels l2, l4 curve slightly from the nominal arcuate path which the plane of each panel follows. Thus on opposite sides of an aperture 16, the panels 12, 14 are distorted in opposite directions, although joining to form solid segments on each of the inner and outer surfaces of the body 10.

Alternatively, as shown in FIG. 4, the distortion of individual panels may be substantially reduced by increasing the number of flutes 17, but decreasing the thickness of each flute. The total enclosed internal volume may thereby be kept substantially the same without the introduction of a possible point of weakness due to a sharp discontinuity in the notch region at which the panels providing the opposition faces of an aperture join to form a solid wall.

A different arrangement is provided to a like effect though the use of shortened filler panels 19 in the regions of the apertures, as shown by the enlarged fragmentary sectional view of FIG. 5. The shortened filler panels 19 are separated by the desired flutings extending through the multi-ply structure, but do not disrupt the normal long conductive path between the exposed and cold side surfaces.

Another alternative expedient in accordance with the in vention is also shown in FIG. 5, and comprises the use of a filler material 20, such as a low density foam, or a high density material having low heat conductivity in the aperture. Either inserted material contributes to the solidity of the structure, but if a filler is employed, it is preferred to use a low density material, partly for insulation properties but particularly because the benefits of weight reduction are substantially fully retained.

It may be observed that for a given amount of material the included apertures I6 effectively increase the thickness of the nozzle 10. There is no conductive medium in these included volumes, and the only heat conduction path is along the lengths of the panels I2, I4. Accordingly, a greater heat capacity is obtained for a given weight of material. It is found that the interior apertures are physically reinforced adequately by the solid ribs existing between the panels 12, 14. These ribs can in any event be modified in thickness and disposition to provide desired final properties.

Through the use of structures in accordance with the invention, improvements of the order of 20 percent (decrease) in the composite density of a particular unit have been achieved, utilizing identical fiber materials and resin systems. The improvement in density reduction is equivalent to like improvements in weight, or heat sink capabilities, whichever it is preferred to utilize. Nonetheless, the surface presented for erosion remains as dense and ablation resistant as a completely solid structure.

Although the specific forms shown have the configuration of hollow bodies of revolution, it will be appreciated that the principles of the invention may also be applied to planar surfaces, or bodies other than bodies or revolution. Where a body of revolution has an extremely large radius, of course, the configuration of an incremental area may be regarded as essentially planar. Planar members are utilized as heat shields, micrometeorite shields, and the like, and correspond directly to a flattened section of the arrangement illustrated in FIGS. 1 and 2.

As shown in the successive steps of FIG. 6-8, the layup and processing of multi-ply structures in accordance with the invention do not require special equipment or special handling techniques. First, as best seen in FIG. 6, the panels are initially overlaid circumferentially within a female mold 22, with the individual panels being cut in a desired geometric shape, which will typically be somewhat irregular, in accordance with the thickness of the body, the degree of flair and taper in the body, and the angle desired for the panel within the body. In this FIG., as in the other FIGS. in the drawings, the representation of the panels has been simplified for clarity. It will be appreciated that many more panels are utilized than are shown. As the layup proceeds, the panels are disposed in groups. Solid plugs or inserts 24 of flattened bandlike form are placed Ion gitudinally along the panels 12 between each adjacent pair of panel groups during the layup. A release agent may conventionally be applied to the surfaces of the inserts 24. Retention means (not shown), such as adhesive strips, staples, wire brads, and the like, may be used temporarily in order to hold the panels and inserts in place. At this point, of course, the structure is not unified inasmuch as the panels 12, 14 are only loosely retained relative to each other and the inserts 24.

In the next step, as shown in FIG. 7, the body is densified and cured by any conventional pressurizing and heating means while remaining within the female mold 22. A hydroclave or autoclave is conventionally used for this purpose. A pressure bag or bladder is placed within the body and covering the exposed surfaces of the panels 12, I4. Hydrostatic pressure (or atmospheric if a vacuum is used) forces the bag 25 constantly outwardly toward the female mold 22 during the densification and cure step. Typically the panels l2, 14 will comprise resin impregnated fabrics of silica, glass, carbon or graphite materials. In the densification step. the excess resin is forced out, the panels l2, 14 are compressed together into a unified mass packed tightly about the inserts and the resin thus forms a hardened cured matrix for the fiber reinforcement.

After densification and curing, as shown in FIG. 8, the inserts 24 are removed to provide the fiuter apertures 16 extending longitudinally along the body. The inserts 24 may, for example, be made of solid strips of Teflon" which, because of its extremely low coefficient of friction, does not adhere and permits ready mechanical withdrawal of the strips 24. Alternatively, any suitable chemically decomposable material may be employed.

Destructible inserts for this purpose may be formed through the use of a suitable material, such as a plaster which can withstand the heat of the densification step and retain sufficient rigidity, even though the plaster can later simply be washed out with water. It is also feasible to use soft material, such as a soft aluminum alloy. The use of a hardened rubber insert, however, is preferred because of the relatively low cost of this material, and because of the ease with which such material can be machined and removed from a formed part.

As a final step for the completion of the component itself, as partially shown in FIG. 8, the exposed surface is machined and finished to given dimensions. The remainder of the structure may be machined, overlaid with other materials, or filament windings, or inset into a mating shell as desired.

As shown in FIG. 9, the internal apertures provide integral conduits for an appropriate cooling medium. Under particularly extreme conditions, the heat capacity of a given ablative structure may be exceeded, either because of the intensity of the exposure or because of the duration of the exposure. It has accordingly been the practice, in liquid cooled rocket engine systems, to achieve a degree of cooling by piping liquid fuel past the combustion chamber and the nozzles to cool this portion of the engine while at the same time preheating the fuel for immediate combustion. The system of FIG. 9 is an example of this type of heat exchange system. Liquid fuel from the tank is passed through conduits 32 and through the apertures 16 in the nozzle 10 before passage to the combustion chamber 34. This arrangement preheats the fuel while protecting the nozzle It].

The arrangement of FIG. 9 provides merely one illustration of the use of fluted apertures for handling a cooling medium. As will be recognized by those skilled in the art, a cooling medium need not be circulated through the aperture from a fuel supply or other low temperature source, but a suitable fluid medium might be stored within the apertures, with or without external cooling by means of a heat exchanger.

While there have been described and illustrations of various structures and methods in accordance with the invention, it will be appreciated that the invention is not limited thereto, but includes all modifications and variations encompassed within the scope of the appended claims.

lclaim:

I. An ablative structure comprising a plurality of panels of high temperature and ablation resistance material each having opposite edges and bonded together and extending in aligned side-by-side relation along substantially similar paths wherein the opposite edges respectively define a first surface of said structure and an opposite surface of said structure, the opposite edges of each panel being transversely displaced with respect to each other in said structure, certain of said panels defining a plurality of spaced apertures out of communication with said first and said opposite surfaces and extending substantially normal to said paths.

2. An ablative structure comprising a plurality of panels of high temperature and ablation resistance material each panel having opposite edges and bonded together to form a wall of given thickness and having opposing surfaces defined by the respective opposite panel edges, the individual panels extending from one surface at least part way to the other along paths slanted relative to said surfaces and being transversely displaced along said surfaces, certain of said panels defining interior apertures extending along said structure.

3. An ablative structure comprising a plurality of thin, elongated resin impregnated fabric panels of high temperature and ablation resistance material permanently bonded together and extending in side-by-side relation along similarly angled curved paths, each panel having opposite edges which collectively define a first surface of said structure and an opposite surface of said structure, certain of said panels cooperating to define a plurality of essentially uniformly spaced apertures out of communication with said first surface, said opposite surface and each other extending along said panels substantially normal to said curved paths.

4. An ablative structure comprising a plurality of panels of high temperature and ablation resistance material bonded together and extending in side-by-side relation along substantially like paths relative to interior and exterior surfaces of the structure and wherein each panel has opposite edges collectively defining an interior and exterior structure surface, the successive panel edges being overlaid along a selected direction on the surfaces, and the paths being angledrelative to the surfaces such that long heat conductive paths exist along the panels relative to the thickness of the structure, the panels being disposed to define interior apertures out of communication with the interior and exterior surfaces and lying along axes substantially normal to the selected direction.

5. An ablative structure for exposure to high temperature, high velocity gases comprising a body of revolution having an inner exposed surface and an outer surface and comprising at least two circumferential wall segments, each formed of a plurality of panels of high temperature and ablation resistance material circumferentially overlapped about the circumference of the wall segment and permanently bonded together, each panel being disposed along an arc length extending from the inner radius of the wall segment to the outer radius of the wall segment and each panel having a geometrical configuration such that an interior edge thereof extends along the length of the wall segment and defines a portion of the inner exposed surface and an exterior edge thereof extends along the length of the wall segment and defines a portion of the outer surface, certain of the panels within each wall segment defining a plurality of substantially like spaced apertures out of communication with each other and with the inner exposed surface of said walled body and the opposite outer surface of said walled body, and extending along said are lengths, the angles of the arc lengths relative to the circumference of the wall segment being alike for all panels of the wall segment throughout substantially the length thereof, and relatively low so that long arc lengths are provided in relation to the radial thickness of the wall segments, the wall segments being disposed with adjacent and substantially matching surfaces and the panels between adjacent wall segments being interleaved and permanently bonded to eachother, whereby a smooth walled integral ablative body is provided.

6. The ablative structure of claim 5 wherein the panels of said body comprise at least two different types of fabric, wherein a plurality of panels of each given type are bonded together to provide layers of the given types wherein the panels of each layer are distorted relative to the nominal arc lengths to define the interior apertures, the apertures defining aperture axes extending along the length of the structure and being elongated fiutings separated by substantial walls formed by the distorted panels.

7. An ablative structure comprising a body of revolution disposed about a longitudinal axis, said body comprising a plurality of resin impregnated panels of high temperature and ablation resistance material permanently bonded together, each panel having opposite edges which collectively define an interior and exterior surface of said body respectively, each panel forming a partial arc within the body and extending along an arc length from interior radial limits, adjacent panels overlapping with a substantial interlaminer shear area along the arc length, certain of said panels being disposed to define a plurality of spaced apertures out of communication with the interior and opposite exterior surfaces of the body of revolution and each other and extending substantially normal to the arc lengths, the panel edges along the interior and exterior surfaces of the body of revolution being substantially parallel to the longitudinal axis.

8. An ablative structure as set forth in claim 7 wherein said body of revolution is formed of at least two discrete wall segments of different materials, panels of which are interleaved and permanently bonded together in a transition zone, the panels being disposed to define the interior apertures through the entire length of the structure.

9. An ablative structure comprising an interior surface of revolution defining a central aperture and having an exterior surface, said structure comprising a plurality of resin impregnated individual panels of high temperature and ablation resistance material permanently bonded together in fixed relation and wherein each panel has opposite edges which define a portion of said interior and exterior surfaces respectively, said panels being aligned in generally side-by-side relation and lying lengthwise relative to the central aperture and in a par tial arc about the central aperture, certain of said panels being separated along an interior portion of the arc length to define a plurality of substantially like spaced apertures out of communication with each other, the central aperture, and the exterior surface of said structure, said spaced apertures lying substantially normal to said individual panels along the entire length of the surface of revolution and the adjacent panels along the edges of the apertures being joined to form solid inner and outer thicknesses of material for the structure, such that the structure provides greater thermal capacity for a given weight of material.

10. The invention as set forth in claim 9 above, wherein the apertures are established by groups of filler panels spaced apart along the arc lengths.

11. The invention as set forth in claim 9 above, wherein the apertures are substantially filled with lightweight heat insulative foam material. 

1. An ablative structure comprising a plurality of panels of high temperature and ablation resistance material each having opposite edges and bonded together and extending in aligned sideby-side relation along substantially similar paths wherein the opposite edges respectively define a first surface of said structure and an opposite surface of said structure, the opposite edges of each panel being transversely displaced with respect to each other in said structure, certain of said panels defining a plurality of spaced apertures out of communication with said first and said opposite surfaces and extending substantially normal to said paths.
 2. An ablative structure comprising a plurality of panels of high temperature and ablation resistance material each panel having opposite edges and bonded together to form a wall of given thickness and having opposing surfaces defined by the respective opposite panel edges, the individual panels extending from one surface at least part way to the other along paths slanted relative to said surfaces and being transversely displaced along said surfaces, certain of said panels defining interior apertures extending along said structure.
 3. An ablative structure comprising a plurality of thin, elongated resin impregnated fabric panels of high temperature and ablation resistance material permanently bonded together and extending in side-by-side relation along similarly angled curved paths, each panel having opposite edges which collectively define a first surface of said structure and an opposite surface of said structure, certain of said panels cooperating to define a plurality of essentially uniformly spaced apertures out of communication with said first surface, said opposite surface and each other extending along said panels substantially normal to said curved paths.
 4. An Ablative structure comprising a plurality of panels of high temperature and ablation resistance material bonded together and extending in side-by-side relation along substantially like paths relative to interior and exterior surfaces of the structure and wherein each panel has opposite edges collectively defining an interior and exterior structure surface, the successive panel edges being overlaid along a selected direction on the surfaces, and the paths being angled relative to the surfaces such that long heat conductive paths exist along the panels relative to the thickness of the structure, the panels being disposed to define interior apertures out of communication with the interior and exterior surfaces and lying along axes substantially normal to the selected direction.
 5. An ablative structure for exposure to high temperature, high velocity gases comprising a body of revolution having an inner exposed surface and an outer surface and comprising at least two circumferential wall segments, each formed of a plurality of panels of high temperature and ablation resistance material circumferentially overlapped about the circumference of the wall segment and permanently bonded together, each panel being disposed along an arc length extending from the inner radius of the wall segment to the outer radius of the wall segment and each panel having a geometrical configuration such that an interior edge thereof extends along the length of the wall segment and defines a portion of the inner exposed surface and an exterior edge thereof extends along the length of the wall segment and defines a portion of the outer surface, certain of the panels within each wall segment defining a plurality of substantially like spaced apertures out of communication with each other and with the inner exposed surface of said walled body and the opposite outer surface of said walled body, and extending along said arc lengths, the angles of the arc lengths relative to the circumference of the wall segment being alike for all panels of the wall segment throughout substantially the length thereof, and relatively low so that long arc lengths are provided in relation to the radial thickness of the wall segments, the wall segments being disposed with adjacent and substantially matching surfaces and the panels between adjacent wall segments being interleaved and permanently bonded to each other, whereby a smooth walled integral ablative body is provided.
 6. The ablative structure of claim 5 wherein the panels of said body comprise at least two different types of fabric, wherein a plurality of panels of each given type are bonded together to provide layers of the given types wherein the panels of each layer are distorted relative to the nominal arc lengths to define the interior apertures, the apertures defining aperture axes extending along the length of the structure and being elongated flutings separated by substantial walls formed by the distorted panels.
 7. An ablative structure comprising a body of revolution disposed about a longitudinal axis, said body comprising a plurality of resin impregnated panels of high temperature and ablation resistance material permanently bonded together, each panel having opposite edges which collectively define an interior and exterior surface of said body respectively, each panel forming a partial arc within the body and extending along an arc length from interior radial limits, adjacent panels overlapping with a substantial interlaminer shear area along the arc length, certain of said panels being disposed to define a plurality of spaced apertures out of communication with the interior and opposite exterior surfaces of the body of revolution and each other and extending substantially normal to the arc lengths, the panel edges along the interior and exterior surfaces of the body of revolution being substantially parallel to the longitudinal axis.
 8. An ablative structure as set forth in claim 7 wherein said body of revolution is formed of at least two discrete Wall segments of different materials, panels of which are interleaved and permanently bonded together in a transition zone, the panels being disposed to define the interior apertures through the entire length of the structure.
 9. An ablative structure comprising an interior surface of revolution defining a central aperture and having an exterior surface, said structure comprising a plurality of resin impregnated individual panels of high temperature and ablation resistance material permanently bonded together in fixed relation and wherein each panel has opposite edges which define a portion of said interior and exterior surfaces respectively, said panels being aligned in generally side-by-side relation and lying lengthwise relative to the central aperture and in a partial arc about the central aperture, certain of said panels being separated along an interior portion of the arc length to define a plurality of substantially like spaced apertures out of communication with each other, the central aperture, and the exterior surface of said structure, said spaced apertures lying substantially normal to said individual panels along the entire length of the surface of revolution and the adjacent panels along the edges of the apertures being joined to form solid inner and outer thicknesses of material for the structure, such that the structure provides greater thermal capacity for a given weight of material.
 10. The invention as set forth in claim 9 above, wherein the apertures are established by groups of filler panels spaced apart along the arc lengths.
 11. The invention as set forth in claim 9 above, wherein the apertures are substantially filled with lightweight heat insulative foam material. 